Local Anesthetics

Snapshot

• A 36-year-old man presents to the emergency department with a wound laceration of the hand. He is a construction worker that accidently cut himself with his tools approximately 2 hours prior to presentation. His most recent tetanus vaccination was given 2 years ago. There does not appear to be any possible contamination or foreign body at the site. The wound extends beyond the dermis. Wound irrigation is commenced and a local anesthetic is administered prior to suturing.

Introduction

• Local anesthetics
  • prevent sensory nerve impulses from reaching the central nervous system (CNS)
    • this is accomplished by blocking the inner portion of the sodium channel 
      • which in turn prevents the propagation of an action potential
      • most effective in rapidly firing neurons
  • structure
    • a lipophilic group is joined to a hydropilic group via
      • an amide or ester linkage
        • biotransformation of amides mainly occur in the liver
          • tertiary amine local anesthetics cross membrane in uncharged form and
            • undergo ionic change in order to bind to sodium channel in charged form
        • biotransformation of esters are accomplished by plasma cholinesterases (pseudocholinesterase)
  • onset and duration of action
    • influenced by
      • tissue pH
        • infection can decrease pH (more acidic) in the affected tissue
          • alkaline anesthetics will therefore become charged, which
            • impairs its ability to penetrate the membrane to block sodium channels
              • more anesthetic would be needed
      • lipid solubility of the drug
      • drug concentration
      • nerve morphology
  • effects
    • local anesthetics can result in vasodilation, which
      • causes the drug to diffuse away from the site of action
        • vasoconstrictors (e.g., epinephrine) can correct this
          • thus enhancing the local action of the drug
  • order of nerve blockade
    • small-diameter fibers > large diameter fibers and myelinated fibers > unmyelinated fibers 
      • size predominates over myelination
  • order of loss
• (first) pain → temperature → touch → pressure (last) 

Medications

• Medication
  • esters
    • benzocaine, cocaine, procaine, and tetracaine
  • amides
    • bupivacaine, lidocaine, and mepivacaine
• Clinical use
  • minor surgical procedures
  • spinal anesthesia
• Adverse effects
  • CNS symptoms
    • excitation or depression
  • cardiovascular toxicity (bupivacaine)
  • arrhythmia
  • methemoglobinemia (benzocaine)